1. Field of the Invention
The present invention relates to an optical device using a diode laser as a light source and in particular concerns an optical system which can be applied to an apparatus for recording and/or reproducing optical information, video or audio or both, on or from an optical recording medium formed on a disc with a high density by means of a small size spot of a concentrated laser light beam produced by a diode laser. For example the invention is applicable to an optical video disc apparatus.
2. Description of the Prior Art
In general, the semiconductor laser has advantageous features in that the laser beam can be directly modulated without the need for using an optical modulator and the laser device can be implemented in a highly miniaturized size as compared with other lasers. Thus, the semiconductor laser is increasingly employed in various instruments and apparatus as a light source. When the diode laser is to be used as the light source for an optical recording and reproducing apparatus, it must be capable of producing an output light beam having a reasonably high energy level in correspondence with the optical sensitivity of a recording material. For example, where the recording material is a layer of silver salt deposited on a film and thus necessarily subjected to developing and fixing treatments after exposure to a recording illumination, the output power of the semiconductor laser may be relatively low. However, for a recording medium such as metallic thin film or pigment film, recording energy level in the range of 100 to 1000 mJ/cm.sup.2 is required as measured on the recording medium. Thus, the output power of the semiconductor laser used as the light source has to be on the order of several tens of milliwatts. In order to assure the recording energy level of the magnitude described above on the recording medium, it is desirable that light transfer efficiency should be increased as much as possible in the optical path extending between the diode laser and the recording medium by concentrating the laser beam so as to produce a fine or minute light spot on the recording medium. In this connection, it is noted that the output beam of a diode laser has inherently a relatively large diameter. Thus, concentration or focusing of the laser beam to a diameter as small as possible is required for fulfilling the requirements of concerning the recording energy level described above and at the same time for attaining the recording of a signal at a possibly increased density.
The cross-sectional shape of the light beam at the light emitting plane of a diode laser exhibiting a high output power is related to a plane of a junction of the diode laser and takes an extremely elongated profile having dimensions of several .mu.m in the direction perpendicular to the junction plane and several tens .mu.m in the direction parallel to the junction plane. Further, the light beam emitted from the light emitting plane exhibits a relatively large angular extension, e.g. angular extension of .+-.15.degree. to 20.degree. in the direction perpendicular to the junction plane and .+-.5.degree. to 10.degree. in the direction parallel thereto. Consequently, the semiconductor laser has to be handled in a completely different manner as compared with He--Ne gas lasers or the like.